Platelets Flashcards
Describe how Megs make/releases platelets.
When the meg reaches a certain number of nuclear divisions, it releases 2000-5000 platelets
*stimulated by TPO
What is endomitotic synchronous nuclear replication?
replication scheme of megs in which the replicate their nucleus many times but do not divide their cytoplasm
What proportion of platelets does the spleen normally sequester?
1/3 or 30%
What percentage of total body platelets can be sequestered in the spleen of a pt with an MDS or lymphoproliferative disorder?
60-90%
What is the open canalicular system?
invaginations of the platelet plasma membrane
*inc surface area of the platelet
The submembranous filaments in a platelet allow ________
the platelet to contract and expand
*actin and myosin filaments
What is found in the dense granules?
- nucleosides/ADP (NOT available for metabolism)
- Calcium
- serotonin
What is found in the alpha granules?
growth factors (PDGF) B-thromboglobulin PF4 fibrinogen factor V VWF fibronectin heparin agonist thrombospondin
What are the platelet specific proteins?
B-thromboglobulin
PF4
What is the dense tubular system
similar to ER
Where calcium is stored
What are the 3 types of granules in platelets?
lysosome
dense
alpha
Describe the formation of the primary hemostatic clot
- GpIb binds vWF to adhere platelets endothelium
- platelets contact collagen become activated
- degranulation
- aggregation
What mediates plateet adhesion to subendothelium?
GpIb binding vWF
What is the role of GpIIb/IIIa?
binds fibrinogen to link the platelets together
What is the consequence of nonfunctional GpIIb/IIIa?
Glanzmans –> bleeding
normal platelet count
platelets can do everything but aggregate
(High or low) levels of cAMP inhibit platelets
high
T or F: GpIIb/IIIa binds fibrin
F: fibrinogen (which is cleaved into fibrin)
What is the biochemical effect of an agonist/stimulus binding a platelet surface receptor and activating PLC?
Ca released from dense tubule which activates phospholipase A2 to make TXA2 from arachidonic acid
What happens on the surface of the platelet (in terms of coag cascade)? Why does this need to happen on a platelet surface and not just freely in the blood?
8a is embedded in the platelet surface and forms a complex with 9a which together acts to activate 10
*if this reaction had to happen freely in the blood these substrates (8a, 9a, and 10) would rarely ever meet. This ensures they are all brought together in close proximity at the right time
What are the 2 most potent activators of platelets?
thrombin and TXA2
What effect does inc PGI2 have on platelets?
inhibits platelet aggregation
activates adenylate cyclase to make cAMP which inactivates platelet
Why must must a LOW dose aspirin be taken for desired therapeutic effects (as opposed to a higher does)?
If the dose is too high the cycloxygenase in the endothelial cell will also be inhibited. if the endo COX is inhibited, then platelets become activated/aggregate (via dec PGI2 –> adenylate cyclase inactivation –> dec cAMP–> inc activ)
With low does aspirin, only the COX in the platelet is inhibited –> dec TXA2 –> activation of adenylate cyclase –> inc cAMP –> dec aggregation
What does bleeding or DIC do to the number of platelets?
decreases
What are the 2 major causes of splenic sequestration?
alcoholic cirrhosis and portal hypertension
*much more common than lymphoma and leukemia
What is the c-MPL receptor?
TPO receptor on Megs
What are the 4 causes of thrombocytopenia?
- failure of production
- inc consumption (bleeding DIC)
- sequestration
- dilution
What are immune related causes of thrombocytopenia?
idiopathic infection (esp HIV) Drug: Heparin, quinidine post transfusion feto-maternal alloimmune thrombocytopenia
What do the Abs in ITP often recognize?
GpIIb/IIIa
What is HIT?
Heparin-plasma protein complex binds Ab which binds platelet.
Complement is deposited and platelet-drug-Ab complex removed
What molecule regulates platelet production?
TPO
What is the treatment for chronic ITP?
corticosteroids
What are 2 non-immune related causes of platelet destruction?
DIC and TTP
What are the 4 major causes of reactive thrombocytosis
trauma
surgery
blood loss
chronic iron deficiency
Why does essential thrombocytopenia progress to MF?
platelets release growth factors (from alpha granules…PDGF?) that stimualte fibroblasts
